Surface Plasmon Resonance (SPR) and Microgravimetric sensing techniques, such as Quartz Crystal Microbalance, are known independently as methods suitable for in-situ, label free sensing and analysis of binding reactions. Sensors using SPR or QCM have been used to analyse biological, biochemical and chemical samples.
QCM devices comprise a quartz crystal wafer having two planar metal electrodes disposed on the two surfaces of the wafer. The sample to be analysed is adsorbed onto the surface of one of the electrodes. The shift in the quartz crystal can be excited to mechanical resonance by an alternating an electric field due to the inverse piezoelectric effect. The resonance frequency is dependent upon the mass of material adsorbed onto the surface electrode. For instance, the resonance frequency decreases for mass accumulation and increases for mass reduction. The shift in frequency can be related to the adsorbed mass using analytical equations. A mass loading in the order of about 1 ng/cm2 can be detected.
SPR is a known method for the detection of chemical changes occurring at the surface of a thin metal film. SPR measures changes in the optical thickness (i.e. refractive index) arising from molecular adsorption on the metal surface. In SPR, an evanescent wave (which is an exponential-decaying wave) exists at the sensor surface. In what is known as the Kretchmann geometry, an evanescent wave is generated when total internal reflection of incident light occurs at the interface of a substance with a high refractive index and a substance of low refractive index (i.e. a glass-air interface of a prism). SPR occurs under certain conditions when a thin film of metal (e.g. gold or silver) is placed on the surface. When the incident light is monochromatic, the free electrons of the metal will oscillate (ie. a surface plasmon is excited) and absorb energy corresponding to a certain angle of incident light. The angle is called the SPR angle. The SPR signal is detected by measuring the intensity of the reflected light. At the SPR angle, a sharp decrease or “dip” in intensity is measured as the surface plasmon absorbs energy.
The position of the SPR angle depends on the refractive index of the sensing surface which changes upon the binding of molecules to the surface. Consequently, the SPR angle changes according to the amount of molecules bound to the surface. The detection limitation of SPR is approximately 1 ng/cm2.
SPR and QCM techniques each have their own specific strengths, weaknesses and have assumptions that are inherent in data collection and analysis. Accordingly, each technique is sensitive to different properties of a thin film sample.
Analytical devices that utilise both SPR and QCM techniques are known. German patent DE 10024366 discloses an analytical devices that combines SPR and QCM using a grating coupler. The use of a grating coupler requires that the incident light beam pass through the sample solutions for SPR, requiring the flow-cell and the sample be optically transparent. This has the disadvantage of the results having a low signal/noise ratio. Moreover the SPR measurement sensitivity is low compared to prism-coupler SPR sensors.
There is a need to provide a sensor and an analytical technique, which overcomes, or at least ameliorates, one or more of the disadvantages described above.